Process for controlling the addition of an auxiliary fuel

ABSTRACT

The invention relates to a process for controlling the addition of an auxiliary fuel to combustion of at least one combustible material in a combustion chamber. In addition, the invention relates to the use of the process, in which the concentration is measured in the flue gas of at least one pollutant formed by incomplete combustion. In a second step, the rate of the auxiliary fuel fed to the combustion is increased when the concentration of the at least one pollutant in the flue gas exceeds a predetermined upper limiting value, or is decreased when the concentration measured of the at least one pollutant falls below a predetermined lower limiting value. Measurement of the concentration and setting the rate of the auxiliary fuel is repeated until the measured concentration of the at least one pollutant in the flue gas is in a range between the lower and upper limiting values.

The invention relates to a process for controlling the addition of an auxiliary fuel to combustion of at least one combustible material in a combustion chamber. In addition, the invention relates to the use of the process.

In particular in the combustion of combustible materials which burn only unsatisfactorily independently, it is necessary to maintain the combustion by addition of an auxiliary fuel. Conventional processes are, for example, thermal disposal of solid or liquid waste materials and also thermal exhaust gas purification.

In order to achieve complete combustion without further pollutants being formed in the combustion, it is necessary to carry out the combustion within preset temperature ranges. The temperature is currently set customarily via the amount of the added auxiliary fuel. Thus, for example, it is known from VDI Guidelines VDI 2442, March 2006, pages 41 to 55, to measure the temperature of the combustion in the exhaust gas and on the basis of the temperature to control the feed of natural gas as auxiliary fuel.

In addition to the temperature, it is also customary to measure the oxygen content of the flue gas. By determining the oxygen content, the amount of combustion air or any other desired oxygen-comprising gas which is required can be set. In the event of an excessive oxygen content in the flue gas, the rate of oxygen-comprising gas which is fed to the combustion is reduced, and when the oxygen content in the flue gas falls below a preset value, the rate is increased.

Generally, pollutants in the flue gas are suppressed provided that preset control temperatures and control oxygen values are maintained. By this means, emissions can be kept at prescribed limiting values or fall below them. Customarily, the control temperature must be of a height such that the emissions significantly fall below the limiting value of the pollutants, and the limiting value is generally never achieved. Under differing operating states of a combustion system, the set value is customarily not changed. This leads to the fact that under a normal operating mode, the control temperature is higher than is actually required. This leads, in particular, in the case of substances which burn only inadequately independently, to the fact that the demand for auxiliary fuel is higher than is actually necessary.

In a combustion process without addition of an auxiliary fuel, it is known, for example, from DE-A 101 53 643 to determine quantitatively oxidizable fractions present in the exhaust gas. At the same time, the burner flame is monitored. For this, the temperature is detected. It is further described that in the case of incomplete combustion the burner is shut off and the combustion system is set to fault condition.

EP-A 0 697 564 discloses determining the fraction of oxygen and hydrogen and/or carbon simultaneously in the exhaust gas of a fired system. On the basis of the content of hydrogen and/or carbon monoxide in the exhaust gas, the air feed is controlled. Incomplete combustion in this case can result from an air excess or air deficit. In the event of an air excess, the combustion temperature falls below a necessary value, in case of an air deficit the oxygen is insufficient in order to carry out complete combustion.

A process and a device for controlling and monitoring combustion are likewise disclosed by EP-A 0 697 565. In this case, likewise the fraction of pollutants in the exhaust gas is measured and on the basis of the pollutant content the air feed to the burner of the fired system is set. In this case the fraction of carbon monoxide in the exhaust gas is measured. The air feed is increased for reduction of the carbon monoxide fraction. Suitable sensors which are suitable for measuring the oxygen content and/or carbon monoxide content in a gas stream are disclosed, for example, by U.S. Pat. No. 5,549,871 or DE-U 202 00 373.

Said documents merely disclose, in the case of unburnt components in the flue gas, increasing or decreasing the oxygen feed.

It is an object of the present invention to provide a process for controlling the addition of an auxiliary fuel to a combustion in which the rate of the auxiliary fuel to achieve complete combustion can be minimized.

The object is achieved by a process for controlling the addition of an auxiliary fuel to combustion of at least one combustible material in a combustion chamber, which process comprises the following steps:

-   (a) measuring the concentration in the flue gas of at least one     pollutant formed by incomplete combustion, -   (b) increasing the rate of the auxiliary fuel added to the     combustion when the concentration in the flue gas of the at least     one pollutant measured in step (a) exceeds a predetermined upper     limiting value or decreasing the rate of the auxiliary fuel added to     the combustion when the concentration of the at least one pollutant     measured in step (a) falls below a predetermined lower limiting     value, -   (c) repeating the steps (a) and (b) until the concentration measured     in the flue gas of the at least one pollutant is in a range between     the lower and upper limiting value.

In an alternative embodiment, the object is achieved by a process for controlling the addition of an auxiliary fuel to combustion of at least one combustible material in a combustion chamber, which process comprises the following steps:

-   (a) setting a preset value for the combustion temperature, wherein     the preset value is of a level such that complete combustion of the     at least one combustible material proceeds, -   (b) measuring the combustion temperature and increasing the rate of     the auxiliary fuel added to the combustion if the measured     temperature falls below the preset value of the combustion     temperature and decreasing the rate of the auxiliary fuel added to     the combustion if the preset value of the combustion temperature is     exceeded by the measured temperature, -   (c) measuring the concentration of at least one pollutant formed in     the flue gas by incomplete combustion and lowering the preset value     of the combustion temperature, -   (d) repeating steps (b) and (c) until the measured concentration of     the at least one pollutant formed by incomplete combustion exceeds a     predetermined limiting value, -   (e) increasing the preset value of the temperature and repeating     steps (b) and (c) until the measured concentration of the at least     one pollutant formed by incomplete combustion is again below the     predetermined limiting value.

The at least one combustible material can be solid, liquid or gaseous. Customarily, the combustion is carried out at a temperature of above 700° C. with an oxygen excess. This means that more oxygen is fed to the combustion plant than is theoretically necessary in order to ensure complete oxidation of all materials participating in the reaction. As a result, unconsumed oxygen remains in the flue gas of the combustion plant. Addition of the auxiliary fuel is required, in particular, when the solid, liquid or gaseous material fed to the combustion plant has a heating value which is low such that the required combustion temperature is not achieved. In order to maintain the combustion reaction, therefore the addition of a fuel having a high heating value is necessary.

The oxygen necessary for the combustion is generally added in the form of air. However, it is also possible to feed any other desired oxygen-comprising gas. For example, pure oxygen or oxygen-enriched air can also be fed. Since an excessive stream of oxygen-comprising gas leads to a decrease of the combustion temperature, it is desirable to carry out the combustion with a low oxygen excess. Customary preset values for the residual oxygen content in the flue gas are generally between 0 and 11% by volume. The oxygen content in the flue gas can be determined using any desired sensor known to those skilled in the art. Preference is given to use of a zirconium-dioxide-base sensor, since this has a rapid reaction time.

In a preferred embodiment, in the process according to the first alternative, in addition, the temperature of the flue gas or the combustion is also measured. In order to achieve complete combustion, the temperature is customarily between 700 and 1000° C. If, for example, combustible substances comprising exhaust gases are burnt in the thermal exhaust gas purification having a high oxygen content, the required temperatures are customarily in the range between 770 and 830° C. In the case of hydrocarbonaceous substances, required temperatures of usually 780° C. to 850° C. are set. Required temperatures in the combustion of halogenated materials are generally in the range from 850 to 950° C.

The necessary combustion temperature is customarily set by the addition of the auxiliary fuel and the rate of the oxygen-comprising gas which is fed to the combustion.

In particular in the combustion of combustible materials having a low heating value, the temperature of the combustion is set by increasing or decreasing the rate of the auxiliary fuel added to the combustion. By increasing the rate of the auxiliary fuel added to the combustion the temperature of the combustion is increased, whereas it is reduced by reducing the amount of the auxiliary fuel added to the combustion. The auxiliary fuel in this case customarily has a heating value which is higher than the heating value of the combustible material which is burnt in the combustion.

Generally, the at least one pollutant in the flue gas which is formed by incomplete combustion is carbon monoxide, at least one hydrocarbon or at least one unburnt or incompletely burnt component of the at least one combustible material and/or of the auxiliary fuel or a mixture thereof. In particular, the at least one pollutant is carbon monoxide or a hydrocarbon.

In addition to said pollutants, however, the concentration of any other pollutant which can be present in the flue gas can also be determined.

It is an advantage of the process according to the invention that the rate of the auxiliary fuel can be reduced compared with the temperature control known from the prior art by determining the pollutant content in the flue gas. In particular, a reduction may be achieved compared with processes known from the prior art in which the rate of the auxiliary fuel is set on the basis of the combustion temperature.

The concentration of the at least one pollutant in the flue gas can be measured immediately downstream of the combustion chamber or downstream of an apparatus arranged in the flue gas channel. In the measurement immediately downstream of the combustion chamber, also, the measurement is customarily performed in the flue gas channel. However, it is also possible to measure the concentration of the at least one pollutant already in the combustion chamber directly after the combustion.

Apparatuses arranged in the flue gas channel are, for example, apparatuses for cooling or purifying the flue gas. Generally, heat exchangers are used for cooling the flue gas. Such a heat exchanger can be used, for example, for steam generation also. The steam thus produced may be used, for example, as heating steam for processes of the most varied types. Apparatuses which are used for purifying flue gas are, for example, scrubbers or filters.

At least one combustible material which is burnt using the addition of an auxiliary material generally does not burn in a self-sustaining manner. This means that the temperature required for combustion cannot be maintained by combustion of the combustible material alone. The reason for this is, for example, that the combustible material is a composition of matter which, in addition to combustible substances, also comprises incombustible substances. Generally, the combustible material comprises organic compounds which are oxidized in the combustion. The incombustible materials which are present are, for example, inert gases such as noble gases or nitrogen in the case of gaseous combustible materials or water in the case of pasty or liquid combustible materials. Water can also be present as water vapor in gaseous combustible materials.

Generally, the at least one combustible material which is fed to the combustion is waste, sludge comprising combustible substances, waste water comprising combustible substances or waste gas comprising combustible substances.

Wastes which are burnt are, for example, domestic refuse, hazardous waste or industrial waste and also mixtures thereof.

Sludges are taken to mean generally solid materials which are dispersed in water, wherein the water content is customarily high enough that the sludge is pasty.

Liquids comprising combustible substances are, for example, liquid materials or waste water from chemical production, from painting plants, drying plants, or else from printers. The combustible substances which are present in the liquid can be liquid or solid. If the at least one combustible material is solid, this is generally in the form of small particles which are dispersed in the liquid. Liquid substances are, for example, solvents or oils. These can be mixed in water, or else be present in anhydrous form.

If the combustible material is a waste water comprising combustible substances, it is preferred to reduce the water content before the combustion. For this, use can be made of any desired drying or distillation process known to those skilled in the art. Alternatively, it is also possible, for example, to remove the water by extraction. In the drying and/or distillation processes, the water is vaporized and in this manner the at least one combustible substance is concentrated. As a result, the content of combustible substances and thereby the heating value is increased. An advantage of this is that the rate of auxiliary fuel to be fed can be reduced.

If the combustible material is in gaseous form, it is generally an exhaust gas. The exhaust gas in this case comprises at least one combustible material in any concentration. Exhaust gases which are subjected to combustion originate, for example, from painting plants, drying plants, any chemical processes, printers or fermentations. In addition to the combustible materials, generally a large fraction of inert gases are present in the exhaust gas so that the exhaust gas does not burn in a self-sustaining manner. This is the case, in particular, when the fraction of combustible substances falls below a minimum concentration necessary for combustion. Waste gases comprising odor-intense materials can also be subjected to thermal exhaust gas purification.

The auxiliary fuel to be fed to the combustion customarily has a higher heating value than the combustible material which is to be burnt. Suitable auxiliary fuels can be solid, liquid or gaseous. Preferably, the auxiliary fuel is selected from coal, coke and liquid or gaseous hydrocarbons.

If the auxiliary fuel is a solid, for example coke or coal, it is preferably ground before addition to the combustion.

In one embodiment of the invention, in addition to the concentration of the at least one pollutant in the flue gas, the oxygen content in the flue gas is also determined. By this means the rate of oxygen-comprising gas which is fed may be adapted to the rate of combustible material to be burnt and auxiliary fuel. In this manner at an excessive oxygen content in the flue gas, the rate of the oxygen-comprising gas fed to the combustion can be reduced and at an insufficient oxygen content in the flue gas, the rate of oxygen-comprising gas fed to the combustion can be increased. In this manner, even at an elevated demand for auxiliary fuel it is possible to ensure that complete combustion proceeds. If the rate of oxygen-comprising gas is not adapted to the rate of auxiliary fuel or combustible material, it is otherwise possible that, owing to oxygen deficit, incomplete combustion proceeds or that, owing to an excessive gas stream of oxygen-comprising gas, the combustion is too greatly cooled and the combustion temperature falls below the value required for complete combustion.

The process according to the invention is used for controlling the addition of an auxiliary fuel to a combustion preferably in a refuse incineration plant or a plant for thermal exhaust gas purification. In this case not only plants for combustion of domestic refuse, but also plants for combustion of hazardous waste or industrial wastes are termed refuse incineration plants.

Thermal exhaust gas purification is used, for example, when insufficient solvent mass flow rates in the exhaust gas do not enable economic operation of an adsorptive or absorptive recovery system, if, in addition to organic solvents, other pollutants such as, for example, phenols, plasticizers, age resistors or paint mists or dusts are present in the exhaust gas, the selective separation of which requires high expenditure and which leads to irreversible damage of the absorption medium used for recovery. In addition, thermal exhaust gas treatment is also used if, in the exhaust gas, a mixture of numerous organic solvents is present, the composition of which can change during the adsorption/desorption process owing to differing adsorbability of the individual components and which, for renewed setting of the exact formula, must be decomposed into its various components. Also, if separation of the solvent from the desorption medium and/or absorption medium is not possible or economic, thermal exhaust gas purification is preferred. In addition, thermal exhaust gas treatment is carried out if, in the adsorption/desorption process, unwanted side reactions lead to corrosive products or if at the same time liquid wastes are to be disposed of and only one plant for disposal will be erected.

One embodiment of the invention is shown in the drawing and is described in more detail in the description hereinafter.

The sole FIGURE shows a process chart of the process according to the invention.

A combustible material which is to be burnt is fed to a combustion chamber 1 via a first line 3. The combustible material can be solid, liquid or gaseous. Customarily, solid or liquid wastes or exhaust gases are fed to the combustion chamber 1. Generally, the heating value of the combustible material fed via the first line 3 is not high enough in order to carry out combustion at a temperature necessary for complete combustion. For this reason, an auxiliary fuel is fed to the combustion chamber 1 via a second line 5. A suitable auxiliary fuel is any fuel which is known to those skilled in the art, the heating value of which is higher than the heating value of the combustible material which is fed via the first line 3. Suitable auxiliary fuels are, as already mentioned hereinbefore, for example, coke, coal and liquid or gaseous hydrocarbons.

The oxygen necessary for a combustion is fed to the combustion chamber 1 via a third line 7. The oxygen in this case can be fed as pure oxygen or in the form of an oxygen-comprising gas. An oxygen-comprising gas customarily used is, for example, air. In addition, it is also possible to use, for example, oxygen-enriched air.

The rate of the fuel fed to the combustion chamber 1 is set via a first valve 9 which is arranged in the second line 5. The rate of the oxygen-comprising gas fed to the combustion is set via a second valve 11 which is arranged in the third line 7.

Via a flue gas line 13, the flue gases formed in the combustion chamber 1 during the combustion are removed. In the embodiment shown here, the oxygen content in the flue gas formed in the combustion is measured by a first sensor 15 in the flue gas line 13. Any desired sensor known to those skilled in the art by which the oxygen content can be determined is suitable as first sensor 15.

Via a control line 17, the first sensor 15 is connected to the second valve 11 via which the rate of oxygen-comprising gas which is fed to the combustion chamber 1 is controlled. The rate of oxygen-comprising gas which is fed by the third line 7 to the combustion chamber 1 is set via the second valve 11 as a function of the oxygen content measured in the flue gas. At an excessive oxygen content in the flue gas, the rate of oxygen-comprising gas is reduced by reducing the throughflow cross section of the second valve 11, whereas at an insufficient oxygen content in the flue gas, the rate of oxygen-comprising gas which is fed is increased by further opening the second valve 11. Any desired controllable valve known to those skilled in the art is suitable as second valve 11. Suitable valves are, for example, those having a slide. In this case, for example, suitable valves are valves having a gate. These include, for example, valves having a parallel slide gate valve, disk closing gate valve, wedge gate valve, double disk parallel slide gate valve or double disk wedge gate valve. In addition, suitable valves are those having a rotary disk valve or a flap valve.

The content of at least one pollutant is measured in the flue gas by a second sensor 19. The pollutant determined by the second sensor 19 is, for example, carbon monoxide, at least one hydrocarbon or other unburnt components in the flue gas. According to the invention, the second sensor 19 is connected to the first valve 9 via a second control line 21, via which the rate of auxiliary fuel is set. The second valve 9 can in this case be any type known to those skilled in the art.

If the content of pollutants in the flue gas which is measured by the second sensor 19 is too high, the first valve 9 is further opened in order to increase the rate of an auxiliary fuel which is fed. If, in contrast, the fraction of pollutants measured in the flue gas falls below a predetermined lower limiting value, the first valve 9 is further closed in order to reduce the rate of auxiliary fuel. In this manner the combustion can always be carried out with the required minimum amount. This leads to an optimal utilization and reduced consumption of auxiliary fuel compared with control systems known from the prior art.

The upper limiting value for the concentration of at least one pollutant measured by the second sensor 19 is customarily less than or at most exactly the same as the permitted limiting value for this pollutant. The lower limiting value is customarily less than the upper limiting value and at least zero. If the lower limiting value is chosen as zero, this means that a concentration of the at least one pollutant which is measured by the second sensor 19 is below the limit of detection, when this sensor displays no value. At a lower limiting value equal to zero, customarily the rate of auxiliary fuel added via the second line 5 is reduced until an occurrence of the pollutant measured by the second sensor 19 is measured. As soon as an occurrence is measured, the rate of auxiliary fuel which is fed is again slightly increased, so that the combustion is carried out in such a manner that the rate of pollutant lies below the limit of detection.

In addition to the embodiment shown here, in which the first sensor 15 and the second sensor 19 are two separate components, it is also possible to use a combined sensor which can detect both the oxygen content and also the content of at least one pollutant. In addition, it is also possible to arrange the first sensor 15 and the second sensor 19 downstream of the combustion region in the combustion chamber 1.

In an embodiment in which further apparatuses, for example apparatuses for cooling the flue gas or for further purification of the flue gas, are present in the flue gas line 13, the first sensor 15 and the second sensor 19 can also be positioned downstream of such an apparatus which is arranged in the flue gas line 13.

In addition to the first sensor 15 by which the oxygen content is measured in the flue gas line 13 and the second sensor 19 by which the concentration of at least one pollutant resulting from incomplete combustion is determined in the flue gas, the temperature in the flue gas or in the combustion can also be determined. The desired combustion temperature may then likewise be set, for example, by the rate of the auxiliary fuel which is added. For instance, generally, a higher rate of auxiliary fuel leads to a higher combustion temperature.

In particular, if a preexisting combustion plant is intended to be modified, wherein the existing plant customarily controls the rate of auxiliary fuel which is to be fed on the basis of the combustion temperature, it is possible by means of the second sensor 19 to determine the content of at least one pollutant in the flue gas. If pollutants in the flue gas are detected by the second sensor 19, the required temperature at which the combustion is carried out is increased. This leads to the fact that the rate of auxiliary fuel which is to be fed is likewise increased. If no pollutants are detected in the flue gas by the second sensor 19, or the amount of pollutants is below the lower limiting value, the required temperature at which the combustion is carried out is reduced. This simultaneously leads to a reduction of the rate of auxiliary fuel which is to be fed. In this manner, the combustion may always be carried out at the lowest suitable temperature.

EXAMPLES Comparative Example 1

In a chemical process, a product is produced by partial oxidation of a hydrocarbon by a mixture of oxygen and nitrogen. In the partial oxidation, byproducts form which are fed together with the nitrogen to a system for thermal exhaust gas purification. In addition, reaction water which is produced in the production which is loaded with organic substances is fed to the combustion.

The rate of auxiliary fuel to be fed is set via a temperature controller. As auxiliary fuel, natural gas is used. The temperature is selected in such a manner that even with fluctuating fraction of byproducts and organic substances in the material stream fed to the combustion, complete combustion is carried out. The rate of natural gas required for the combustion is 9 m³ per 1000 m³ of exhaust gas.

Example 1

In the combustion corresponding to comparative example 1, the temperature controller is replaced by measurement of the carbon monoxide content in the flue gas and control of the rate of the auxiliary fuel on the basis of the monoxide content. In contrast to the temperature control according to comparative example 1, the natural gas consumption can be lowered to 7 m³ per 1000 m³ of exhaust gas.

Comparative Example 2

Exhaust air from a drying process is fed to a combustion with regenerative preheating in order to oxidize organic components present in the exhaust air. The rate of auxiliary fuel required, in this case natural gas, is set via a temperature controller. The consumption of natural gas is 3 m³ per 1000 m³ of exhaust gas.

Example 2

In the process according to comparative example 2, the temperature controller is replaced by measurement of the carbon monoxide content in the flue gas and setting the rate of the auxiliary fuel on the basis of the carbon monoxide content. By this means, the natural gas consumption was lowered, compared with the temperature controller, to 2.5 m³ per 1000 m³ of exhaust gas.

LIST OF REFERENCE SIGNS

-   1 Combustion chamber -   3 First line -   5 Second line -   7 Third line -   9 First valve -   11 Second valve -   13 Flue gas line -   15 First sensor -   17 Control line -   19 Second sensor -   21 Second control line 

1-15. (canceled)
 16. A process for controlling the addition of an auxiliary fuel to the combustion of at least one combustible material in a combustion chamber, comprising: (a) measuring the concentration in the flue gas of at least one pollutant formed by incomplete combustion, (b) increasing the rate of the auxiliary fuel added to the combustion when the concentration in the flue gas of the at least one pollutant measured exceeds a predetermined upper limiting value or decreasing the rate of the auxiliary fuel added to the combustion when the concentration of the at least one pollutant measured falls below a predetermined lower limiting value, (c) repeating the measuring and increasing and/or decreasing until the concentration measured in the flue gas of the at least one pollutant is in a range between the lower and upper limiting value.
 17. The process according to claim 16, wherein, in addition, the temperature of the flue gas is in a range between the lower and upper limiting values.
 18. The process according to claim 17, wherein the temperature of the combustion is set by increasing or decreasing the rate of the auxiliary fuel added to the combustion.
 19. The process according to claim 16, wherein the at least one pollutant is carbon monoxide, at least one hydrocarbon or at least one unburnt or incompletely burnt component of the at least one combustible material and/or of the auxiliary fuel or a mixture thereof.
 20. The process according to claim 16, wherein the at least one pollutant is carbon monoxide.
 21. The process according to claim 16, wherein the concentration of the at least one pollutant in the flue gas is measured downstream of the combustion chamber or downstream of an apparatus arranged in the flue gas channel.
 22. The process according to claim 21, wherein the apparatus arranged in the flue gas channel is used for cooling or purifying the flue gas.
 23. The process according to claim 21, wherein the apparatus arranged in the flue gas channel is used for steam generation.
 24. The process according to claim 16, wherein the at least one combustible material is not combusted in a self-sustaining manner.
 25. The process according to claim 16, wherein the at least one combustible material is waste, sludge comprising combustible substances, liquids comprising combustible substances, or exhaust gas comprising combustible substances.
 26. The process according to claim 16, wherein the auxiliary fuel is selected from coal, coke and liquid or gaseous hydrocarbons.
 27. The process according to claim 16, wherein the oxygen content is additionally determined in the flue gas.
 28. The process according to claim 27, wherein in the case of an excessive oxygen content in the flue gas, the rate of oxygen-comprising gas fed to the combustion is reduced and in the event of an insufficient oxygen content in the flue gas, the rate of oxygen-comprising gas fed to the combustion is increased.
 29. The process according to claim 16 used in a refuse incineration plant or a plant for thermal exhaust gas purification.
 30. A process for controlling the addition of an auxiliary fuel to combustion of at least one combustible material in a combustion chamber, comprising: (a) setting a preset value for the combustion temperature, wherein the preset value is of a level such that complete combustion of the at least one combustible material proceeds, (b) measuring the combustion temperature and increasing the rate of the auxiliary fuel added to the combustion if the measured temperature falls below the preset value of the combustion temperature and decreasing the rate of the auxiliary fuel added to the combustion if the preset value of the combustion temperature is exceeded by the measured temperature, (c) measuring the concentration of at least one pollutant formed in the flue gas by incomplete combustion and lowering the preset value of the combustion temperature, (d) repeating said measuring of combustion temperature and concentration until the measured concentration of the at least one pollutant formed by incomplete combustion exceeds a predetermined limiting value, (e) increasing the preset value of the temperature and repeating said measuring of combustion temperature and concentration until the measured concentration of the at least one pollutant formed by incomplete combustion is again below the predetermined limiting value.
 31. The process according to claim 30, wherein the at least one pollutant is carbon monoxide, at least one hydrocarbon or at least one unburnt or incompletely burnt component of the at least one combustible material and/or of the auxiliary fuel or a mixture thereof.
 32. The process according to claim 30, wherein the at least one pollutant is carbon monoxide.
 33. The process according to claim 30, wherein the concentration of the at least one pollutant in the flue gas is measured downstream of the combustion chamber or downstream of an apparatus arranged in the flue gas channel.
 34. The process according to claim 33, wherein the apparatus arranged in the flue gas channel is used for cooling or purifying the flue gas.
 35. The process according to claim 33, wherein the apparatus arranged in the flue gas channel is used for steam generation.
 36. The process according to claim 30, wherein the at least one combustible material is not combusted in a self-sustaining manner.
 37. The process according to claim 30, wherein the at least one combustible material is waste, sludge comprising combustible substances, liquids comprising combustible substances, or exhaust gas comprising combustible substances.
 38. The process according to claim 30, wherein the auxiliary fuel is selected from coal, coke and liquid or gaseous hydrocarbons.
 39. The process according to claim 30, wherein the oxygen content is additionally determined in the flue gas.
 40. The process according to claim 39, wherein in the case of an excessive oxygen content in the flue gas, the rate of oxygen-comprising gas fed to the combustion is reduced and in the event of an insufficient oxygen content in the flue gas, the rate of oxygen-comprising gas fed to the combustion is increased.
 41. The process according to claim 30 used in a refuse incineration plant or a plant for thermal exhaust gas purification. 